This invention relates to a technique of improving the adhesion or binding properties of a diamond-like thin film with respect to slightly adhesive substrates of articles, such as metallic mold substrates.
"Diamond-like film" or "diamond-like thin film" used herein is defined as an amorphous carbon film having a Raman's absorption at about 1,550 cm.sup.-1. It should be noted that the diamond-like film is clearly different from diamond film because the latter has a sharp Raman's absorption peak at 1,333 cm.sup.-1 and is an aggregate of micro crystals. Diamond-like film does not have Raman's absorption peak 1,333 cm.sup.-1 and usually such diamond-like film or diamond-like thin film has a Vickers hardness Hv of at least 5,000 kg/mm.sup.2 (refer, e.g., to TRC News, January 1987, Vol. 6-1, page 7, published by Torey Research Center and Journal of Applied Physics of Japan, Vol. 55, No. 7 (1986), page 640).
Methods of manufacturing diamond-like thin films by the vapor phase process are varied (refer, e.g., to HYOMEN KAGAKU (Surface Chemistry), vol. 5, No. 108 (1984), pp. 108-115 "Various methods"). Diamond-like thin films are extensively used as corrosion- and abrasion-resistant protective films over the surfaces of variously shaped articles that need to be protected.
The diamond-like thin films are capable of being bound solidly to substrates of silicon or the like. However, they are not as adhesive to certain types of article substrates and have a common problem of easily coming off from those substrate surfaces upon subjection to external forces. The shortcoming makes the films unable to be fully effective as protective coatings for applications where corrosion resistance or wear resistance is of essential importance. In particular, Fe metals and alloys (e.g., soft steel "STC", stainless steel, and hardened steels "SKD", "SKS"), alloys of other metals such as Co and Ni, glass, and ceramics are known to produce weak bonds between themselves and diamond-like thin films. Iron-based substrates, such as structural and sliding parts, are of the highest industrial utility. Glass and ceramics too have a broad range of applications including sliding members of thermal heads. It is therefore important to improve the adhesiveness of diamond-like thin films to these substrate surfaces on which they are to be formed.
Pretreatment of such substrates has been taught, e.g., by Japanese Patent Application Public Disclosure Nos. 200898/1985, 204695/1985, and 174376/1986.
The molds or dies for injection molding, extrusion, compression molding, etc. of glass and plastics have hitherto been made from cemented carbides. The materials are expensive and require much time and cost to procure and fabricate into the objects. Susceptibility to cracking due to the lack of toughness is another disadvantage. The brittleness of the cemented carbides has to be compensated for and their abrasion resistance be enhanced. To this end, it has been proposed to coat the frictional or sliding surfaces of metallic molds protectively with diamond-like thin films, e.g., by Japanese Patent Application Public Disclosure Nos. 15169/1990, 22012/1990, and 15170/1990. However, the diamond-like thin films do not bind firmly enough to the metallic mold substrate surfaces and, being aggregates of microcrystals, they easily separate from the mold surfaces by dint of external forces. They, therefore, have not proved fully satisfactory as protective coatings for applications where resistance to corrosion and abrasion is essential.
Hardened steels, on the other hand, are available at lower cost and do not require much time and cost for fabrication but the resulting mold surfaces are worn faster and hence have shorter life. The drawback could be overcome by coating the surfaces with a diamond-like thin film formed by a vapor phase process, as proposed, e.g., by the above-mentioned Patent Application Public Disclosure Nos. 15169/1990, 22012/1990, and 15170/1990. However, by the same token, the binding force is insufficient. There are other approaches to the manufacture of diamond-like thin films (refer, e.g., to HYOMEN KAGAKU (Surface Chemistry), vol. 5, No. 108 (1984), pp. 108-115 "Various methods"). Those methods generally require as high a substrate temperatures as 600.degree. C. or upwards, which can anneal the hardened steels and impair the hardness of the resulting molds.
Patent Application Public Disclosure No. 200898/1985 recommends ion etching of a Co-WC alloy substrate surface by direct action of glow discharge before a diamond-like thin film is formed on the surface as a high-hardness film. Since no accelerating voltage is applied, the etching efficiency is not adequately high for the purpose of enhancing the adhesion. Thus the improvement in adhesion to which the present invention is directed is not satisfactorily achieved. Patent Application Public Disclosure No. 204695/1985 likewise aims at an increase in the film-forming rate. The end is attained by introducing Ar gas into a reduced-pressure chamber, applying a voltage across positive and negative electrodes to produce a plasma, and then subjecting a substrate to the plasma action. The plasma ion concentration being low, the etching effect is rather limited for the improvement of adhesion. Public Disclosure No. 174376/1986 intends to improve the adhesion of substrates by treatment with plasma gas and then by oxidation treatment to form an oxide coating. The plasma requires diffusion in the first place so that the positive ions can pass through the positive-potential grid. This makes it impossible for a sufficient amount of positive ions to form a film to reach the substrate, thereby rendering the process inefficient. The prior art methods thus have failed to produce a diamond-like thin film with adequately high bond between the film and the substrate. Patent Application Public Disclosure No. 80190/1991 teaches bombardment of a substrate surface with an accelerated ion beam. The technique is advantageous over those described already but is still unable to bring an adequately enhanced adhesion.
Patent Application Public Disclosure No. 174508/1984 sets out the ionization evaporation technique that is utilized in the present invention. The reference specification describes that a thin film of Si, Ti or the like is formed as an intermediate layer over a basis metal plate of Ni, Cu, Fe, Co or the like, and then a diamond-like film is formed thereover to provide a Vickers hardness of about 5000. However, the bases of bulk materials such as iron and steel that contain Fe, Co, etc. and stainless steel are not adequately receptive to the application of the coatings, and the coating film of Si or the like does not achieve satisfactory adhesion strength.
U.S. Pat. No. 4,753,414 (to McCandless) uses RF plasma in forming a carbonaceous coating film over a base. According to a paper written by the inventor and cited in the patent specification, the RF plasma method produced a Vickers hardness of only about 1850. The patent process, therefore, is unable to yield the diamond-like film of the present invention. In addition, the same patent is silent on any intermediate layer.
On the other hand, U.S. Pat. No. 5,112,025 claims that a diamond film can be directly formed on a plating film of Ni, with allegedly ample adhesion. However, the fact is the adhesion is "ample" in the sense that the plating film is capable of withstanding a molding pressure of at most about 400 kg/cm.sup.2. Apart from this, Ni is originally a hardly adherent metal.
U.S. Pat. No. 4,490,229 (to Mirtish) teaches activating a base by bombardment with neutral Ar to increase the adhesion of a diamond-like film to the base surface. In the absence of accelerating means for the bombarding gas, however, the activation of the base surface is not sufficient for attaining full adhesion, and hence the hardness is unsatisfactory.
None of these printed publications of the prior art suggest that the use of an amorphous Si-C mixture as an intermediate layer makes it possible to form an excellently adherent and hard diamond-like film on a bulk metal such as stainless steel or steel containing Fe, Co, etc.
In view of the above, we proposed in Patent Application 142678/1991 the use of an Mo intermediate layer for added adhesiveness. The intermediate layer was found to produce very great binding power. We tried other substances for the intermediate layer and attained results comparable to or even better than those of the patent application.
It is therefore an object of the present invention to provide articles protected with a diamond-like thin film which is strongly bound to and highly adhesive to the substrate and exhibits improved peeling resistance and durability.
Another object of the invention is to manufacture highly wear-resistant metallic molds from inexpensive hardened steel. Hardened steel has such low thermal resistance and poor adhesion to diamond-like thin films that they have been unable to lend themselves to the industrial manufacture of diamond-coated metallic molds with adequate wear resistance. Patent Application No. 214913/1989 discloses a technique of forming a diamond-like film by bombardment of a substrate with Ar ions or the like and subsequent ionization evaporation to provide protection for metals and ceramics. It does not teach, however, the application of the technique to the fabrication of metallic molds from hardened steel.